A conventional solder composition (solder paste) used in a surface mounting technique is in the form of a paste which is obtained by dispersing, in a flux, solder powder as Sn-Pb alloy, and mixing necessary additives therein.
To mount an electronic device on a circuit board using a solder composition, the solder composition is put on a pad which serves as a mounting portion, and the device is placed on the board such that the terminal of the device is positioned on the solder composition. Then, the resultant structure is heated to melt the solder composition and electrically connect the device to the pad.
In the above structure, the device can be electrically connected to the pad in a reliable manner if they are sufficiently separated from each other. However, where they cannot sufficiently be separated in a high-density mounting, it is possible that portions of solder are connected to each other at the time of putting the solder composition on the pad or melting the same, thereby forming a solder bridge. When the solder bridge has been formed, short-circuiting will occur which degrades the circuit function. Moreover, repairing the solder bridge requires high cost.
As a solder-precipitating composition free from the above problem, U.S. Pat. No. 5,145,532 (Jpn. Pat. Appln. KOKAI Publication No. 1-157796) discloses a solder-precipitating composition which contains Sn powder and an organic acid salt of Pb.
This solder-precipitating composition has the following solder deposition mechanism:
1) The organic acid salt of Pb is separated into organic acid ions and Pb ions by heating;
2) Pb ions are displaced with nonionic Sn which has an ionization tendency higher than Pb, with the result that Pb ions are reduced to nonionic Pb and part of nonionic Sn is oxidized to Sn ions; and
3) The reduced Pb is dispersed into the other part of Sn (i.e. its nonionic portion), thereby forming a solder.
It was found that the above-described solder precipitation process shows behaviors which significantly differs from those shown in the case of melting solder at its melting point. For example, in the case of pre-coating pads of a printed circuit board with solder, using the conventional solder composition, adjacent portions of solder paste may be connected to each other after heating unless an appropriate amount of solder paste which is proportional to the area of the pad is supplied, thus forming a solder bridge. On the other hand, in the case of the solder-precipitating composition disclosed in U.S. Pat. No. 5,145,532, it is not necessary to supply the solder-precipitating composition by the printing method using a stencil corresponding to a pad pattern. This solder-precipitating composition is characterized in that even if the solder-precipitating composition is supplied on the overall surface of the board, solder is coated only on conductive pads, and no solder bridges are formed. This feature has now been highlighted under the circumstances that high-density circuit boards have been developed in accordance with downsizing of electronic parts. In such high-density circuit boards, it is extremely difficult to supply a solder composition by the printing method so as to avoid occurrence of solder bridges. In the solder-precipitating composition of U.S. Pat. No. 5,145,532, however, the above-described solder precipitation mechanism exerts on each Sn particle. It has been found that in Sn powder having a certain particle size distribution, the smaller the diameter of a particle and the higher the surface area ratio of the same, the higher the reaction speed (in other words, the larger the diameter, the lower the reaction speed). In light of this, the solder-precipitating composition of U.S. Pat. No. 5,145,532 inevitably shows variations in a final or target solder composition. This means that it is difficult to perform uniform pre-coating of solder. Sn powder of a uniform particle size may well cause uniform reaction. However, preparation of such Sn powder costs too much.
Moreover, in the case of obtaining an alloy consisting of 63 wt % Sn and 37 wt % Pb only by displacement reaction between Sn powder and Pb ions in the organic acid salt of Pb, the amounts of those portions of nonionic Sn and the organic acid salt of Pb, which are used only in ion exchange reaction and are not finally converted into a solder alloy, increase in the solder-precipitating composition. Accordingly, the amounts of effective components decrease in the solder-precipitating composition.
As aforementioned, in the conventional soldering method for soldering an electronic device, a solder composition is supplied on the pads of a printed circuit board by the printing method, and then the electronic device is mounted on the solder composition. Thereafter, the solder composition is made to reflow to solder the lead portions of the device. In addition to this soldering method, a method is put to practice, in which an electronic device is mounted after solder is pre-coated on the pads of a board and then a flux is supplied thereon, thereby heating the pre-coated solder to make it to reflow and fix the lead portions of the device. At the time of mounting an LSI electronic device such as QFP (Quad Flat Package), it is necessary to pre-coat solder such that the pre-coated solder layer has a thickness sufficient to absorb variations in lead coplanality between the leads of the component, in order to reliably fix the leads to the pads provided on the same surface of the board.
However, in the case of pre-coating the pads of a printed circuit board with a great amount of solder, using the solder-precipitating composition of U.S. Pat. No. 5,145,532, it is necessary to use a thick mask which makes printing control difficult, to supply the solder-precipitating composition on the board by the printing method as a general method. Furthermore, even if a great amount of solder-precipitating composition is supplied to form a thick solder layer on the board, the solder-precipitating composition inevitably melts and flows out during heating, with the result that the pads cannot be pre-coated with a great amount of solder. Therefore, it is necessary to repeatedly supply the solder-precipitating composition on the board and repeatedly subject it to substitution reaction.